OBD2 Codes

Chevrolet P0420 – Meaning, Causes, Symptoms, & Fixes

P0420 catalyst system efficiency below threshold bank 1
Reading Time: 2 minutes


Chevrolet Code P0420 Definition

Catalyst system efficiency below threshold (Bank 1).

What Does P0420 Mean For Your Chevrolet?

The function of the catalytic converter is to break down harmful pollutants that are created by your Chevrolet during the combustion cycle and convert them into harmless gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by your Chevrolet.

Chevrolet P0420 Symptoms

  • Check Engine Light is on
  • Lack of power from the engine
  • Decreased fuel economy
  • Rotten egg or sulfur smell

Chevrolet P0420 Causes

  • Faulty oxygen sensor
  • Worn and internally failing catalytic converter
  • Leak in exhaust system

Chevrolet Code P0420 Severity – Moderate

While P0420 does not pose a danger to the driver and may not produce any symptoms with issues related to drivability, it can cause severe damage to other components of your Chevrolet if not addressed in a timely manner. It is recommended that you address P0420 as soon as possible to keep the cost of the repair to a minimum.

Chevrolet Code P0420 Common Diagnosis Mistakes

Do not overlook other codes that are paired along with P0420. Chevrolet codes P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0174, and P0171 can give false data readings that create conditions which trigger code P0420. If any other codes are present, they should be addressed first.

Chevrolet P0420 Diagnosis

Tools Needed to Diagnose:
  1.  Scan your Chevrolet to verify P0420 is the only code present. If other codes are present, they must be addressed first.
  2. Examine the exhaust system for damage and leaks. Pay close attention to the exhaust manifold, gaskets, and exhaust pipes pre-catalytic converter. If any leaks are found, repair the leak, clear the code, and complete several drive cycles to verify that was the fix.
  3. With the Chevrolet running and at normal operating temperature, use the digital multimeter to check the voltage reading of the downstream O2 sensor. The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V if the catalytic converter is functioning properly. If the voltage of the downstream O2 sensor is constantly jumping between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to be replaced.


You may also like


  1. My 2009 chevy colorado cranks over but will not start. It shut down while driving. Hooked it up to a obd2 scanner and got (code PO420) bank 1. Nothing else pulls up. Could this be a oxygen sensor?

    1. Ever find out the issue, I just had the same thing happen to me

  2. PO420 bank1 1 code popped up on obd2 scanner on my 2009 chevy colorado. I was driving and it shut down without warning. It will not start back up. Could this be an oxygen sensor? There are no other codes showning

  3. Can you check for spark at the spark plugs?

Leave a reply

Your email address will not be published. Required fields are marked *

More in OBD2 Codes